Rapid Reversible Oxidative Addition of Group 14−Halide Bonds to Platinum(II):  Rates, Equilibria, and Bond Energies

The reversible oxidative addition reactions of methyl(halogeno)tin and methyl(halogeno)germanium compounds to electron-rich platinum(II) complexes of the type [PtMe₂(diimine)] have been studied. Complete kinetic and thermodynamic parameters have been obtained by VT ¹H NMR for the reversible oxidative addition of Me₃EX (E = Sn, X = Cl, Br, I) to [PtMe₂(bpy-^tbu₂)] (bpy-^tbu₂ = 4,4‘-di-tert-butyl-2,2‘-bipyridyl) and related compounds, while partial data have been obtained for the reductive elimination of Me₂SnCl₂ from [PtClMe₂(Me₂SnCl)(bpy-^tbu₂)] and for the oxidative addition of Me₃GeCl to [PtMe₂(bpy-^tbu₂)]. UV−visible spectroscopic studies have also yielded equilibrium constants and ΔG° for the reversible oxidative addition reactions of Me₃SnX (X = Cl, Br, I) to [PtMe₂(diimine)]. Thermodynamic studies quantitatively establish the halogen effect on the oxidative addition reactions studied according to the favorability series I > Br > Cl. Kinetic studies clearly point to an S_N2 mechanism for the reactions studied, and this is further supported by the observation of the cationic complex [PtMe₂(Me₃Sn)(bpy-^tbu₂){OC(CD₃)₂}]⁺ at low temperature in acetone-d₆. Extremely large second-order rate constants are observed for the oxidative addition of Sn−X bonds to dimethylplatinum(II) complexes, some being greater than 10⁸ M^-1 s^-1, and it is established that rates follow the series Sn > Ge > Si > C and I > Br > Cl. Estimates have been made of the Pt−MMe₃ bond dissociation energies for [PtXMe₂(MMe₃)(bpy-^tbu₂)], X = halide, and these are 233, 182, and 172 kJ mol^-1 for M = Si, Ge, and Sn, respectively; the values are useful in rationalizing the chemistry of the Pt−M bonds.